Time-lapse seismic surveying or monitoring involves obtaining seismic data of the same part of the subsurface at different times. It allows studying the changes in seismic properties of the subsurface as a function of time due to for example fluid flow through the underground formation or spatial and temporal variation in fluid saturation, pressure and temperature. Seismic data can be combined to generate images that show the changes. The time-lapse seismic surveying technique has applications such as mapping bypassed oil, monitoring injected reservoir fluids such as water, steam and CO2, and estimating fluid-flow heterogeneity related to pressure compartmentalization and the hydraulic properties of faults and fractures. Time-lapse seismic surveying is also called 4-dimensional seismic surveying.
It is well known that repeatability is a key to the success of time-lapse seismic surveying, and that major concerns are source and receiver locations (see for example the article Time-lapse seismic monitoring: Some shortcomings in nonuniform processing, C P Ross and M S Altan, The Leading Edge, June 1997). Other concerns are source and receiver waveform responses and coupling, and various forms of undesired noise.
Applicants have found that there is another factor that adversely affects the success of marine time-lapse seismic surveying, the properties of the sea.
In international publication WO02075363 the effect of so-called surface multiple reflections is discussed. The surface multiple reflections are contributions to the recorded signal caused by multiple reflections of the seismic energy between the surface of the sea and the sea bottom. In this patent application it is explained that the surface multiple reflections do not repeat, so that when subtracting two signals, recorded at different dates, the difference signal contains an unknown contribution from the difference of the surface multiple reflections. Therefore this difference may not be the same as the difference between the subsurface signals (that are free from surface multiple reflections) and thus the difference is not an indication of changes in the subsurface between the date of recording the first signal and the date of recording the second signal.
In order to provide a method of carrying out at sea a time-lapse survey of a target layer in an underground formation in which non-repeatable effects caused by the surface multiple reflections (also known as time-lapse artefacts) can be suppressed in a simple manner, it is proposed in this publication to record the signals at substantially repeating sea conditions. This method comprises:
(a) arranging a seismic sensor system at a predetermined position;
(b) positioning a seismic source at least one location, wherein each location has a predetermined position, and recording for each location the signal from the seismic sensor system in response to a sound wave emitted by the seismic source;
(c) positioning after a predetermined period of time the seismic source at the location(s) of step (b), and recording, when the appropriate sea properties are substantially repeating, for each location the signal from the seismic sensor system in response to a sound wave emitted by the seismic source; and
(d) subtracting the obtained signal from a signal previously obtained to get a difference that is used to detect changes in the target layer as a function of time.
Applicant's co-pending application WO2003087878 (issued as U.S. Pat. No. 6,906,982) proposes a method of carrying out at sea a time-lapse survey of a target layer in an underground formation in which non-repeatable effects caused by the surface multiple reflections can be suppressed in a mathematical way. This method comprises the steps of:
(a) positioning the seismic source at a source location having a predetermined position, and positioning the at least one seismic receiver of the seismic receiver system at a receiver location having predetermined position;
(b) recording for these source and receiver locations for each seismic receiver a set of at least two repeat signals obtained from the seismic receiver in response to a set of at least two sound waves emitted by the seismic source, wherein the at least two sound waves are emitted at different sea states and/or surface conditions but so closely in time that differences in the repeat signals due to changes in the subsurface can be ignored, and measuring the two-way water travel times when recording the repeat signals;
(c) positioning after a predetermined period of time the seismic source and the at least one seismic receiver at the source and receiver locations of step (a), recording for each seismic receiver a monitor signal from the seismic receiver in response to a sound wave emitted by the seismic source, and measuring the two-way water travel time when recording the monitor signal;
(d) assuming a model for the signal that includes the subsurface signal, differences in the subsurface signals due to changes in the subsurface, and the contributions of surface multiple reflections;
(e) determining a set of filter functions that enable eliminating the surface multiple reflections, which filter function(s) are functions of the measured two-way water travel times at the position of the seismic source of step (a);
(f) calculating for each receiver the estimated differences in the subsurface signals from the repeat signals, the monitor signal and the filter function(s), which estimated difference is a measure of changes in the subsurface signal caused by changes in the target layer in the time elapsed between recording the repeat signals and recording the monitor signal; and
(g) outputting the estimated difference.
Both methods described above involve relatively complex calculations and require measurement of two-way water times. Additionally, the method described in WO02075363 requires the sea conditions to be precisely repeated in the base and monitor acquisitions and this is difficult to arrange in marine seismic acquisition. The method described in WO2003087878 requires the measurement of seismic two-way-times, which requires either additional acquisition hardware or an imprecise measurement using the standard acquisition setup. Neither of the above methods are adaptive and thus might leave artefacts due, say, to rapidly varying wave-height.
Therefore there is a need to develop a simple adaptive method to suppress time-lapse artefacts due to the effects of changes in sea state and/or surface conditions and conditions without requiring the measurement of two-way water travel times.